How to use set up transparent background on VSE (alpha file combined with mp4)
As mp4 doesn't support alpha channel, the file you have is a black and white movie that brings the alpha information.
White means matte, while black means transparent.
In Blender compositor (or any other) you can plug it into a color mix node, where black will bring the top input image, white the bottom input.
In VSE you can add a mask modifier to the video strip that needs transparency, targeting the alpha strip.
Alpha strip has to stay under the visible strips.